any way to lower the PSI that the air governor calls for air?

The governor in my truck calls for air when the tanks still have around 112psi in them and since there is still too much air in the signal line the air goes straight out the purge valve on the air dryer. It will do this until the pressure in the tanks drops to around 110psi then the air will dump out of the signal line and allow the purge valve to close and fill the tanks. I have the shut off set at 120-125psi(varies depending on engine load)and just want to try and lower the start up point down to around 100psi.

This is a problem I've been having with this truck since I've owned it and over the time I've replace the following with no solution to the problem: air dryer, governor, load out valve in compressor, and finally the compressor itself. I've had six different mechanics look at the truck plus I've called the Bendix customer support number and nobody can figure out what's going on. at first there were times the purge valve wouldn't close even when the pressure was down to when the low air warning buzzer would come on, but then it stopped doing that and just sputtered until the tanks were down to around 110psi.

I've just learned to live with it, but recently I replaced the height control valve and cut down enough on the air usage that the condition is really amplified since it tanks longer for the tanks to bleed down to stop the sputtering. It's to the point now that if I don't either pump the brakes or grab the blowgun I have in the cab and bleed some air out of the tanks I would have to listen to the air sputtering out the purge valve for miles. Not to mention that it's kinda of embarrassing to leave the truck running while you run an errand and come out and can hear the thing sputtering from across the parking lot.

 

I've tried both ways. I've had so many parts thrown into this truck in the last 2yrs trying to eliminate this problem it's sickening. First time I replaced the complete air dryer(it's an AD9)and governor with no luck. Then mechanic said purge valve might be bad so he warrantied a new purge valve kit, no luck with that. Then I went to a different shop and they wanted to replace the dryer, I said it's new and they said it still might be bad so they got it covered under warranty and I got another new complete dryer. Then I tried a different governor with no luck. Next it was suggested that the load out valve was bad, I got a rebuild kit for that and when I took the old one out it looked fairly new so it seems the previous owner had some issues also. The new load out valve changed nothing. Finally one of the mechanics said the compressor sounded a little noisy and was slow to build air so he suggested a new compressor. So I bought a reman compressor and put it on myself and still no change.

I wish the truck would've acted up during the test drive before I bought it. It waited until I was almost home with it, but I just figured it needed to new air dryer since the truck was 5yrs old and the dryer was still the original. I guess the moral of the story is don't buy a truck from a dealer that is 400 miles away because you can't take it back and make them fix the issues that pop up.

ETA. I tied an air gauge in to the signal line to check pressure levels when the issue started and I can't remember the exact levels(did this last summer)but there was still something like 20-30psi in the signal line when the compressor started trying to fill the tanks and the air would purge out the valve until the psi in the tanks would drop to 110psi then the air in signal line would dump completely out and allow the purge valve to close.

 

I also replaced all the lines going between the governor and air dryer, and replaced the big line going from the compressor to the dryer. I couldn't find any leaks in the lines before hand but decided to change everything out anyway since the lines were cheap.

 

I'll check into that. I'm pretty sure my bags don't have any leaks since I've checked them over really good before, but you never know. I wonder also if a bad height control valve could cause the same issue? The height control valve for the cab is still original and probably needs to be changed.

 

I checked one time and it is coming from the purge valve. I've really been pulling my hair out over this issue. It's probably something stupid that nobody has ever had an issue with(I've had a few odd quirks with this truck that I've never seen before).